This invention relates to cathode ray tubes for producing color imagery and more particularly to an improved system for controlling secondary electrons and improving the color imagery in a color cathode ray tube of the post-deflection acceleration type.
Color cathode ray tubes of the post-deflection type conventionally employ at least one electrode member oriented between the electron gun assembly and the spatially related cathodoluminescent screen. In its simplest form, a color tube of this type is structured to have a color-selection electrode that is spatially positioned relative to the patterned screen. Usually, the post-deflection type of tube construction permits the use of larger openings or apertures in the color-selection electrode than those normally employed in the mask of the conventional shadow mask type of tube. As a result, a screen display of increased brightness is possible in a post-deflection type of tube, but unfortunately problems are also evidenced.
In post-deflection tubes, the color selection electrode is normally operated at an electrical potential of lower value than the potential applied to the screen, the difference in these two potentials determining whether the operation of the tube is fundamentally post-deflection focusing or post-deflection acceleration. During operation of the tube, the scanning primary electron beams strike the solid structural portions of the color-selection electrode thereby effecting the generation of a cloud of secondary electrons in the vicinity of the color-selection electrode. Some of these low velocity secondary electrons are attracted through the large apertures or openings in the color-selection electrode by the higher screen potential, and as a result, randomly impinge the patterned screen. This resultant uncontrolled excitation of the color-emitting phosphors in the screen produces a spurious luminescent haze or background area brightness which markedly degrades the contrast of the screen display. In addition, the primary electron beams impacting the aluminum film on the back of the screen liberate more secondaries, in addition to some reflected primaries, which strike and excite adjacent phosphor areas thereby producing a halo-effect that degrades color purity and further aggravates the problem.
A number of attempts have been made in the art to control and minimize the effects of deleterious secondary emission in the tube. Such control endeavors have included, for example, the positioning of one or more related electrode structures in the proximity of the color-selection electrode; the employment of various configurations of shielding means; the application of carbonaceous or graphite coatings over the surfaces of both the color-selection electrode and the back of the screen; and the deposition of multiple layers of aluminum and other metals, such as boron and boron carbide, over the usual aluminum backing on the screen. In some instances, multi-layered depositions applied to the back of the screen, in thicknesses upwards of 7,000 angstroms, have manifest several drawbacks. Due to excessive thickness and the presence of interfacial surface contamination, multi-layer coatings have been prone to a degree of cracking and peeling in addition to the expensive and time-consuming procedures required for application. Broadly considered, exemplary corrective measures practiced in the art have resulted in varying degrees of success, but in general, most of the results have been less than desired.